The Romans held a festival on December 25 called Dies Natalis Solis Invicti, “the birthday of the unconquered sun.” Many other solar dieties were worshipped collectively on this day, including Elah-Gabal, a Syrian sun god; Sol, the god of Emperor Aurelian and Mithras, a soldiers’ god of Persian origin.
Pic: Alleged representation of Christ in the form of the sun-god Helios or Sol Invictus riding in his chariot.
Emperor Elagabalus (218–222) introduced the festival, and it reached the height of its popularity under Aurelian, who promoted it as an empire-wide holiday.
December 25 was also considered to be the date of the winter solstice, which the Romans called bruma.It was therefore the day the Sun proved itself to be “unconquered” despite the shortening of daylight hours.
When Julius Caesar introduced the Julian Calendar in 45 BC, December 25 was approximately the date of the solstice. In modern times, the solstice falls on December 21 or 22.)
The Sol Invictus festival has a “strong claim on the responsibility” for the date of Christmas, according to the Catholic Encyclopedia.Several early Christian writers connected the rebirth of the sun to the birth of Jesus
It is unknown exactly when or why December 25 became associated with Christ’s birth. The New Testament does not give a specific date.In 245, the theologian Origen denounced the idea of celebrating Christ’s birthday “as if he were a king pharaoh”. He contended that only sinners, not saints, celebrated their birthdays.
The Christmas tree is often explained as a Christianization of pagan tradition and ritual surrounding the Winter Solstice, which included the use of evergreen boughs, and an adaptation of pagan tree worship Extracted from: Wikipedia